The reason why vacuum contactors and vacuum interrupters are used for power switching applications is due to two fundamental properties of the vacuum medium.
First, the dielectric strength of a vacuum gap is far superior to any other switching medium (air in atmospheric pressure, oil, etc.). These properties enable the construction of small sized lightweight practical switching devices.
The tendency of the vacuum arc to cease to exist at low current (known as current chopping), a condition that exists every half cycle in alternating current systems, reduces the scope of application of vacuum switches to systems with low surge impedances (low inductive systems).
The stability of the vacuum arc depends only on the amount of metal vapor present in the gap between the contacts of the vacuum switch at arcing time. The metal vapor is supplied by the contact material. One way to increase the stability of the vacuum arc and reduce the current chopping level is to choose contact material with high vapor pressure. This choice has its limitations because a material with high vapor pressure tends to erode faster and it also might raise the pressure inside the vacuum switch and reduce the interruption capabilities of the device. The other way is to try to prevent the vapor from diffusing into the volume of the switch for a short period of time. This will increase the stability of the vacuum arc and at the same time would not reduce the interruption ability of the vacuum switch. My invention addresses itself to the latter choice. Vacuum means any pressure lower than the standard atmospheric pressure of 760 mm Hg.
Vacuum contactors for motor control applications have been known heretofore.
The vacuum contactor differs from the vacuum interrupter in that its lower voltage rating makes it possible to use a smaller volume. Also, the vacuum contactor has to repeat its function many more times than does the vacuum interrupter.
The requirement for repetitive operation calls for a bellows with a long life.